Auto Service Professional

FEB 2016

Magazine for the auto service professional

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Page 33 of 67

36 | February 2016 probably scored or rusty. Simply replace with new pins and new boots, applying a thin coat of high-tem- perature caliper pin grease prior to instal- lation. Never install guide pins dry. If a caliper is not properly aligned to the rotor, this can result in drag- ging brakes. Caliper misalignment is usually caused by a distorted caliper bracket or severely warped rotors. Master cylinders When you encoun- ter a low, sinking or spongy brake pedal, one suspect is the master cylinder. The cylinder may be bypassing, allowing fuid to leak past the piston seals internally, preventing a pressure buildup. This applies to all vehicles. Master cylinders fea- ture lip cup seals. This design allows for the seal to improve as the pressure increases. Another feature of this seal is that it does not create a vacuum when the piston retracts. As the pedal is depressed, the piston is pushed in the forward direction. The lip cup seal, attached to the primary piston, gener- ates pressure in the primary chamber frst, which in turn, forces the secondary piston forward, building pressure in the secondary chamber. As the pressure increases in both chambers, the edges of the seal are pressed against the bore of the master cylinder. As pressure increases, sealing is improved. In the manufacturing process, the testing procedure for master cylinders is done at low pressure. Lip cup seals are more likely to fail under low pressure. Perform an isolation test. Use the proper tool that won't damage the fexible hydraulic brake hose, clamp off all brake hoses. Press down on the brake pedal, and if the pedal is low or spongy, remove the brake lines from the master cylinder, and install threaded plugs in the outlets (not plastic plugs). Press down on the brake pedal and release the pedal and wait approximately 10 seconds. This allows the quick take-up valve to open The Chassis Calipers that feature pistons on opposing sides, such as the four- piston caliper shown here, feature a fxed mounting position and do not slide on a bracket. If uneven pad wear is found with a fxed- mount caliper, suspect one bank of pistons for sticking issues. Also check the pad backing plate clearance to determine if one pad is dragging on its guides, preventing proper pad alignment and return.

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